A chip and associated circuitry that is responsible for controlling a disk drive. There are different controllers for different interfaces. For example, an IDE interface requires an IDE controller and a SCSI interface requires a SCSI controller. On Macintosh computer systems, the disk controller is built into the motherboard.
A controller for a hard disk is called a hard disk controller (HDC), and a controller for a floppy disk is called a floppy disk controller (FDC).